The invention relates generally to data storage systems and, more particularly, to disk-based data storage systems.
A disk drive is a data storage device that stores digital data in substantially concentric tracks on a data storage disk. During disk drive operation, the data storage disk is rotated about an axis while a transducer is used to read and/or write data from/to a target track of the disk. A servo control loop is used to position the transducer above the target track while the data transfer is taking place. The servo control loop uses servo data read from a surface of the data storage disk as position feedback to maintain the transducer in a substantially centered position above the target track during the data transfer. However, because of positioning errors inherent in the disk drive, the transducer does not remain perfectly centered during the entire data transfer but, instead, remains within a positional range about the target track that is dictated by the mechanical properties of the drive.
When a transducer moves off-track during a write operation, there is a chance that the transducer might inadvertently write data on or near an adjacent track, thus corrupting the data written on the adjacent track. In addition, the data that is written off-track by the transducer may be difficult or impossible to read during a subsequent read operation on the present track due to its off-track position. Thus, an off-track threshold is typically defined in a disk drive that indicates an off-track transducer position beyond which write operations will be suspended. If the transducer goes beyond this off-track position during a write operation, the write operation is suspended until the transducer again comes within the specified positional window about the target track.
The off-track threshold has traditionally been determined during disk drive development based upon collected (worst case) off-track capability (OTC) data and estimates of transducer positioning error. A single off-track threshold value was then used for all transducers within all drives in a production run. During disk drive test, if the OTC of the transducers in a particular drive were all within a specified range and the measured position error of the drive was also within a corresponding range, the disk drive would be passed. It would thus be assumed that the off-track threshold programmed into the drive would be sufficient to prevent adjacent track data corruption and unreadable off-track data. If the OTC of a transducer was not within the specified range, the transducer would not be used in a disk drive. Similarly, if a particular drive displayed greater than a predetermined position error, the drive would not be used. As can be appreciated, the greater the number of units that are left unused during the manufacturing process, the greater the overall manufacturing costs.
Therefore, there is need for a method and apparatus that is capable of increasing yields during the disk drive manufacturing process without compromising disk drive performance.
The present invention relates to a method and apparatus that is capable of increasing yields during the disk drive manufacturing process. Instead of specifying a single off-track threshold value for an entire production run of disk drives, unique off-track threshold values are generated for individual disk drives during the manufacturing process. The off-track threshold values are determined based on the measured off-track capability (OTC) of the transducers actually within each drive as well as the positioning errors associated with each disk drive. Because the off-track threshold values are variable from drive to drive, transducers that were previously discarded as not falling within a predetermined OTC range can now be used as long as they occur in a drive having lower positioning error. Similarly, drives having a large positioning error can be used if paired with transducers having superior OTC. In this manner, manufacturing yields are increased without compromising disk drive performance.
In a preferred embodiment, a separate off-track threshold value is generated for each of the transducers within a manufactured disk drive. In addition, individual threshold values can also be specified as a function of radial position (e.g., on a zone by zone basis) for each transducer. In one approach, a lookup table is provided within the disk drive for storing the off-track threshold values used by the drive. An appropriate value is retrieved from the lookup table for each write operation performed by the disk drive. The off-track threshold values are preferably generated during a test procedure that is part of the manufacturing process. The test procedure is desirably performed after head optimization, channel optimization, and servo calibration procedures have been performed. In a preferred approach, a limited number of possible off-track threshold values are specified for use by the disk drives. By limiting the number of possible off-track threshold values, the off-track threshold determination process is significantly simplified while still achieving the benefits of a variable off-track threshold.